Capacitive touch panels are advantageously thin and light and therefore have gradually replaced the traditional input devices, such as keyboards and mice. As shown in FIG. 1, a conventional two-dimensional capacitive touch panel 10 includes X-axis traces TX1-TX8 and Y-axis traces TY1-TY6. Currently, the touch sensing of the capacitive touch panel 10 is carried out by measuring the self capacitance. For example, when a finger touches a position 12 on the capacitive touch panel 10, the capacitance values of the traces TX8 and TY3 are changed, so it can determine that the finger is at the intersection 12 of the traces TX8 and TY3. However, in multi-touch applications, the sensing method described above is unable to identify the touch points correctly. In FIG. 2, for example, two fingers simultaneously touching the positions 20 and 22 of the capacitive touch panel 10 will change the capacitance values of the traces TX2, TX4, TY2 and TY4, from which there could be sensed two possible pairs of touch points, one of real points 20 and 22, i.e., the positions (TX2, TY4) and (TX4, TY2) where the fingers actually touch, and the other of ghost points 24 and 26, namely positions (TX2, TY2) and (TX4, TY4). This causes the capacitive touch panel 10 unable to properly identify the real points 20 and 22.
FIG. 3 is a circuit diagram of a conventional sensing circuit 30 for a capacitive touch panel, in which a voltage source 32 provides a source voltage Vs applied to a trace TXN of the capacitive touch panel, a switch SW1 is connected between a trace TYM of the capacitive touch panel and a capacitor CL, and a switch SW2 is connected in parallel to the capacitor CL. The switches SW1 and SW2 control charging and discharging of the capacitor CL. When the switch SW1 is on and the switch SW2 is off, in response to the source voltage Vs, the mutual capacitor Cm at the intersection of the traces TXN and TYM will generate a currentI=Cm×dVs/dt  [Eq-1]to charge the capacitor CL to generate a voltage VA, which is converted into a digital signal VD by an analog-to-digital converter 34, and a comparator 36 compares the digital signal VD with a threshold value TH to generate a sensing signal ST to determine whether or not the intersection of the traces TXN and TYM is touched. When the intersection of the traces TXN and TYM is touched, the capacitance value of the mutual capacitor Cm is increased and according to the equation Eq-1, an increase in the capacitance value of the mutual capacitor Cm results in an increase in the current I, which accelerates the rising of the voltage VA and consequently renders the digital signal VD higher than the threshold value TH. Although the foregoing sensing method, which is based on the use of the mutual capacitor Cm, can solve the problem of ghost points in multi-touch applications, it is disadvantageous in that the duration for which the switch SW1 is on, i.e., the on-time of the switch SW1, is under stringent restriction. If the on-time of the switch SW1 is slighter longer than it should be, the digital signal VD may turn higher than the threshold value TH and lead to misjudgment. However, it is extremely difficult to precisely control the on-time of the switch SW1 each time it is turned on.